1. Field of the Invention
The present invention relates to a display device, and in particular relates to a technology effective for implementation in liquid-crystal display devices of active matrix type having TFT elements disposed in a matrix arrangement.
2. Description of the Related Art
Some types of displays intended for use in televisions or personal computers (PCs) have conventionally employed, for example, liquid-crystal display devices of active matrix type. An active matrix type liquid-crystal display device has a liquid-crystal display panel in which liquid crystals are sealed between a pair of substrates, with numerous active elements (also called “switching elements”) disposed in a matrix arrangement on one of the pair of substrates. In most cases, the active elements of the liquid-crystal display device will be TFT elements.
Of the pair of substrates, the substrate on which the TFT elements are disposed in a matrix (hereinafter called the “TFT substrate”) includes the following on the surface of an insulating substrate such as a glass substrate: a plurality of scan signal lines, a plurality of picture signal lines three-dimensionally intersecting the scan signal lines, numerous TFT elements in a matrix arrangement, and numerous pixel electrodes in a matrix arrangement.
In an active matrix liquid-crystal display device, the display area of the liquid-crystal display panel is composed of an array of pixels, each of which having a TFT element and a pixel electrode connected to the source of the TFT element; the gate of each TFT element of multiple pixels arranged in the direction in which the scan signal lines extend will be connected, for example, to a single common scan signal line. Meanwhile, the drain of each TFT element of numerous pixels arranged in the direction in which the picture signal lines extend will be connected, for example, to a single common picture signal line.
The pixel electrode of each pixel, together with the liquid crystals and a common electrode (also called “counter electrode”)), form a pixel capacitor (also called “liquid crystal capacitor”).
Herein, with regard to the drain and source of the TFT elements, a junction with a pixel electrode will be referred to as the source, while a junction with a picture signal line will be referred to as the drain. However, in some instances, conversely, a junction with a pixel electrode is referred to as the drain, while a junction with a picture signal line is referred to as the source.
In an active matrix liquid-crystal display device, it is typical for the gates of multiple TFT elements arranged in the direction in which the scan signal lines extend to be connected to a single common scan signal line as described above. Under these circumstances, the bias condition of the TFT elements connected to a single common scan signal line will change according to the distance from a signal input terminal of the scan signal line. Specifically, the bias condition will differ between TFT elements having shorter distances between their gate and the signal input terminal of the scan signal line, and TFT elements having longer distances between their gate and the signal input terminal of the scan signal line. Such differences in the bias condition of TFT elements may result, for example, in variation of voltage known as “pre-write voltage”, as well as voltage known as “feed-through voltage,” among individual pixels (pixel electrodes).
Specifically, the pre-write voltage and feed-through voltage represent a difference in potential between the voltage of a gradation signal for a given pixel in a picture signal which has been input to a picture signal line, and the voltage which is actually written to the pixel electrode of the given pixel during the ON interval of the gate of the TFT element of the given pixel. If the pre-write voltage and feed-through voltage of individual pixels differ, irregular luminance, flicker, or other such problems become more pronounced, and irregularities in picture quality in a single display device (display panel) may become quite noticeable.
For this reason, in more recent active matrix TFT element liquid-crystal display devices, for multiple TFT elements having a gate connected to a single scan signal line, a method has been proposed for, e.g., varying the size (e.g. the W/L value (the channel width W divided by the channel length L)) of the individual TFT elements according to their distance from the signal input terminal of the scan signal line (refer to, e.g., Japanese Laid-open Patent Publication No. 5-232512 A and Japanese Laid-open Patent Publication No. 9-258261 A).
In the display devices disclosed in the above documents, in view of delay occurring in the scan signal input to the scan signal line, it is proposed for example that multiple TFT elements having a gate connected to a single scan signal line be formed in such a way that TFT elements situated greater distances away from the signal input terminal of the scan signal line will have a progressively larger W/L value. Specifically, in a configuration such as that shown in FIG. 18, for a number of TFT elements Tr1,1, . . . , Tr1,j, . . . . Tr1,M having a gate connected to a scan signal line GL1, the TFT elements Tr1,1 closest to the signal input terminal of the scan signal line GL1 will be the smallest in size, with the size increasing progressively for TFT elements further away from the input terminal of the scan signal line GL1. FIG. 18 is a model circuit diagram illustrating an example of a simplified configuration of a conventional liquid-crystal display panel. In FIG. 18, the triangular symbols at the left ends of the scan signal lines GL1, GLi−1, GLi, GLN−1, GLN denote signal input terminals for scan signals, while the triangular symbols at the upper ends of the picture signal lines DL1, DL2, DLj, DLj+1, DLM denote signal input terminals for picture signals.